The object of this invention is a new method to prepare organic pigment and the new pigment obtained by this method. This pigment is useful especially as a component for increasing whiteness and opacity of various products.
Conventionally, pigments used for increasing whiteness and opacity in paper coating, paints, cosmetic products and for comparable purposes are composed of inorganic materials. Their use impairs recycling of materials, because when the content of the pigments in a material exceeds certain limits, organic material being the carrier or binder of the pigment cannot be burned without a supporting fuel, or without other special arrangements, and the material does not decompose biologically in dumping. Inorganic pigments increase the gravity of the pigmented material and thus freight costs of the final product. Some inorganic pigments contain heavy metals and thus are not applicable in living environment.
Organic pigments have been developed for these purposes mainly based on styrene-butadiene and urea-formaldehyde raw materials, and they have been marketed as latex preparations. These raw materials, too, are combined with environmental difficulties, since they are not decomposed biologically, and for their safe and innocuous burning, high temperatures are necessary. Some latexes marketed function mainly as components giving gloss and without affecting whiteness or opacity. As a white organic pigment, latex composed of hollow particles of styrenebutadiene polymer, such as the product ROPAQUE or Rohm and Haas company, has been marketed. Light scattering of such particles are based on an air bubble in the hollow space, the diameter of which is said to be 0.8 xcexcm. Theoretical calculations have shown, that light scattering from air or gas bubbles in an organic material is the strongest when the diameter of the bubble is of the same order of magnitude as the wavelength of light.
Of the renewable natural raw materials, starch, among others, scatters in dry state light strongly and is sensed white. As with other materials, light scattering is the stronger, the finer the material, and thus stronger for the small-granular than for large-granular starches. So far the common opinion has been that light scattering occurs from the surfaces of the granules. When starch is suspended in water or other liquid, light scattering properties are significantly decreased.
When inorganic pigments are mixed in gelatinized starch, as in paper coating paste, there is a difference on the interface in the refractive index between the pigment and the binding material, and thus light is either reflected or refracted depending on the contact angle. When starch granules are mixed in starch, refractive indices are the same or nearly the same on both sides of the interface, and thus no reflection or refraction occurs on such an interface. Starch granules cannot thus be used as such as pigments in applications, where the binding material is starch, as it is in paper coating. Their pigmenting effect is also weak when mixed in organic liquids such as oils or solvents, due to a small difference in the refractive index.
Starch and starchy materials have been swelled in several industrial operations, for example in cooking extrusion and in popping corn. In these operations, a starchy material containing water is suddenly heated under pressure to temperatures above 100xc2x0 C., and the pressure is suddenly released, causing a swelling of the material due to the water vapour generated. However, at the temperatures and water contents used in these operations starch is gelatinized. The magnitude of pores formed is usually a few millimetres and thus not in the range optimal for light scattering. Since starch in the walls of these bubbles is gelatinized, the bubbles are not stable when in contact with water.
In the method according to U.S. Pat. No. 5,925,380, one or several thermoplastic synthetic monomers with ethene unsaturated bonds are added in starch, and the mixture is heated at temperatures where starch is not gelatinized. The said monomers are polymerized forming hollow particles. Their content is 2-30% of the final product; the particle size is 1-100 xcexcm, and the density in general below 0.1 g cmxe2x88x923. According to these figures, the pores of the smallest particles could be in the size range of the strong light scattering, but there is in the patent no mention of light scattering properties.
Surprisingly it has now been observed, that dry starch particles have sometimes brightly light scattering spots, where the light scattering is manyfold as compared to the surface of a starch granule. Such spots have been observed both in starch samples dried rapidly using the so-called flash drying, and in slowly dried starch samples. The light scattering spot is often in the amorphic centre of the granule, but such spots seem to occur also on the surface of granules. When the sample is contacted with water or another liquid, the light scattering is weakened or disappears, often irreversibly. Especially heating in the presence of large amounts of water leads to disappearing of light-scattering spots. In analogy with the said hollow organic pigments one can assume, that the light scattering would be caused by air bubbles formed or remaining in the granules or on their surface as the granules are dried. For the irreversible disappearance or weakening of the light scattering, two possible reasons seem to be evident. Firstly, when the granules are moistened, the said hollow cavities or air bubbles are filled with water, and material dissolved or suspended in water fills these cavities. Secondly, when the granule is dried, it may shrink in such a way that no new cavity is formed. Correspondingly, heating in the presence of water effects gelatinization of starch, and in this connection a disruption of the granule structure.
In the research on this invention it has now been found, that it is possible to form in starch granules cavities or gas bubbles, which cause a strong light scattering and are stable in contact with water and/or in short-time heating. The amount of the cavities or bubbles can be significantly higher than what is formed spontaneously in drying processes, thus resulting in effective light scattering.